This invention relates to semiconductor lasers, and in particular to semiconductor lasers incorporating means to provide such a laser with an enlarged modal spot size at its output facet.
Primarily for reasons of efficiency, the layer structure of a typical semiconductor laser provides relatively strong waveguiding in the direction normal to the plane of the layers, this direction hereinafter being referred to as the `vertical` direction. In its turn, this strong waveguiding gives rise to a modal spot size with a vertical dimension, typically about 2 .mu.m, that is small in comparison with the modal spot size, typically about 10 .mu.m, of conventional single mode optical fibre.
Some form of modal spot size transformer is therefore required to obtain efficient coupling of light from such a laser into single mode optical fibre.
The problem of launching light efficiently from a semiconductor laser into a single mode optical fibre has some parallels with the related problem of attaining high efficiency optical coupling between a single mode optical fibre and an optical waveguide formed in an integrated optics semiconductor chip.
One form that a spot size transformer can take is that of a microlens. This may be constituted by a lens formed on the end of the fibre, or by a discrete lens element separate from the fibre. The place of the discrete lens can be taken by some other form of discrete modal spot size transformer, or a modal spot size transformer formed integrally with the laser or integrated optics semiconductor chip. Examples of such modal spot size transformers that employ adiabatic tapers appear in the literature. Thus for instances articles by L Morl et al, R Zengerle et al, and G Wenger et al, respectively entitled `Uncladded Thickness Tapers Integrated with InGaAsP/InP Rib Waveguides for Efficient Fibre-Chip Coupling` (Proc. 21st Eur. Conf. on Opt. Comm., ECOC 95, Brussels pp 461-4), `Low-loss Fibre-Chip Coupling by Buried Laterally Tapered InP/InGaAsP Waveguide Structure` (Electronics Lett. Vol. 28, No. 7, pp 631-2), and `Design and Fabrication of Monolithic Optical Spot Size Transformers (MOST's) for Highly Efficient Fiber-Chip Coupling` (J. Lightwave Technology Vol. 12, No. 10 pp 1762-90), each describe a respective example of modal spot size transformer for coupling a single mode optical fibre to a waveguide formed in an integrated optics semiconductor chip. Such transformers have to be precisely aligned with their respective chip waveguides, and the articles relate to transformers in which such alignment is achieved by creating each transformer and its associated chip waveguide in an integrated format.
An article by J Buus et al entitled `Spot Size Expansion for Laser-to-Fibre Coupling Using an Integrated Multimode Coupler (J. Lightwave Technology Vol. 11, No. 4, pp 582-9) describes an alternative form of modal spot size transformer which is neither lens-based nor taper-based. Instead it employs a multilayered coupled waveguide structure using mode dispersion effects in a length of waveguide capable of supporting a limited number of modes. It is stated that the structure is designed for use with waveguides in semiconductor materials including semiconductor lasers, and that the structure is suitable for integration with a semiconductor laser or waveguide, but the authors do not actually describe how such integration can be accomplished. Operation of the structure requires its plane of symmetry of its waveguide layers to be aligned with the centre of the waveguide in the semiconductor chip. This implies a requirement for there to be at least some overlap in height between the chip waveguide layer structure and the transformer layer structure. The provision of such an overlap in an integrated format is complicated by the fact that, when epitaxial regrowth of a planar layer structure is attempted, the planar growth is very liable to be disrupted in the vicinity of any side-wall bounding that growth. Accordingly the integration of the Buus et al transformer with an integrated optics semiconductor ship or semiconductor laser appears a not entirely trivial task.
In the case of coupling a semiconductor laser to a single mode optical fibre using a taper-based modal spot size transformer, the transformer may be incorporated within the optical cavity of that laser, for instance as described by Y Tohmori et al, H Sato et al, M Wada et al, or R Ben-Michael et al, in articles respectively entitled, `Spot-Size Converted 1.3 .mu.m Laser with Butt Jointed Selectively Grown Vertically Tapered Waveguide` (Electronics Lett., Vol 31,m pp 1069-70), `1.3 .mu.m Beam-Expander Integrated Laser Grown by Single-Step MOVPE` (Electronics Lett., Vol. 31, No. 15, pp 1241-2), `Laser Diodes Monolithically Integrated with Spot-Size Converters Fabricated in 2 inch InP Substrates` (Electronics Lett., Vol 31, No. 15, pp 1252-4), and `InP-Based Multiple Quantum Well Lasers with an Integrated Tapered Beam Expander Waveguide` (IEEE Photonics Tech. lett., Vol. 6, No. 12, pp 1412-4). Each of these examples uses a taper for vertical expansion of the laser modal spot size, and for such a taper to be substantially adiabatic (i.e. not to introduce additional loss due to the shortness of the taper) its length needs to be made at least comparable with the length, about 300 .mu.m, of a typical semiconductor laser devoid of any form of modal spot size transformer.